"Frigid weather that has gripped swathes of the northern hemisphere this winter is unusual but does not undermine an overall global trend of warming, experts said on Monday.

Northern Europe and northeast Asia have experienced brutal snowstorms and freezing temperatures, and cities as far south as Sevillein Spain and Miami in Florida have been plunged into the deepest chill in decades.

The reason is a natural phenomenon known as the Arctic Oscillation, said Barry Gromett of Britain's Met Office.

"It's a relatively abnormal pattern but it's not unprecedented at all, it's something that happens every 10 years or so," he said in an interview with AFP."